Modulation of signal transduction emanating from the T cell receptor can be mediated by exogenous cytokines, cell-cell contact, and endogenous metabolites, yielding a specialized functional response to antigenic stimulation. Despite the hostile immunological neighborhood of the intestinal tract, mucosal lamina propria T cells (LPT) remain hyporesponsive to antigens derived from "safe" luminal proteins;yet retain their capacity to mount a vigorous immune response to pathogens. We therefore propose to investigate the mechanisms regulating the maintenance of hyporesponsiveness in mucosal T cells and the environmental signals that enable this normally quiescent mucosal T cell to respond when needed. Signal transduction emanating from the T cell receptor (TCR) is critical for all functional responses, such as proliferation, surface expression of immune mediators such as Fas Ligand, receptors, and adhesion proteins, differentiation, activation induced cell death, and cytokine gene expression. In this proposal we will focus on reactive oxygen species (ROS) derived from incomplete reduction of oxygen, such as hydrogen peroxide, acting as second messengers and thus endogenous modulators of signal transduction emanating from the TCR. We recently reported that the diminished capability of LPT to respond to TCR cross-linking, as measured by weak proliferation, is mirrored by a dramatic loss in their ability to initiate signal transduction from the TCR. To understand this decrease in LPT signaling capacity we investigated the enzymatic pathway directly linked to the a and [unreadable] chains of the TCR, focusing on a potential imbalance between tyrosine kinases and phosphatases associated with the TCR/CD3 complex. Since the plasma membrane is readily permeable to hydrogen peroxide, exogenous administration of low, non-toxic concentrations of H2O2 stimulates a tyrosine phosphorylation pattern that reflects TCR engagement. Similar to what is observed with TCR cross-linking, H2O2-induced tyrosine phosphorylation in LPT is significantly muted. We therefore hypothesized and then reported that the redox status of the hyporesponsive LPT is tilted toward increased reductive capability. In addition, the Crohn's disease-derived (a form of inflammatory bowel disease) LPT responds robustly to TCR ligation (e.g., proliferation, IFN-? production), and the redox status in this inflamed T cell is oxidative, which enables strong signal transduction after TCR engagement. Consistent with our findings it was recently reported that in the normal intestine, lamina propria macrophages do not secrete cysteine, which is required by the LPT to regulate their cytosolic redox potential. These results support the following central hypothesis: The intracellular reducing capacity of the mucosal T cell, which modulates that cell's ability to generate intracellular reactive oxygen species, alters the threshold at which engagement of the T cell receptor initiates an immune response in the normal intestinal mucosa. This hypothesis will be addressed in the following two specific aims: Aim 1. Define the intrinsic defect in the synthesis and/or stability of reactive oxygen species generated in TCR-activated LPT. Aim 2. Establish that cytosolic redox potential sets the threshold for an LPT response. RELEVANCE: To protect us from infections we have a complex system of white blood cells, the principal of these is called the T lymphocyte, which makes strategic decisions for host defense. T cells isolated from the colon or small bowel normally respond very weakly when compared to T cells isolated from blood, while T cells isolated from inflamed intestinal areas in patients with an infection or intestinal cramping and bowel disease due to chronic inflammation respond with still yet another distinct character. This proposal will define those intrinsic properties of the mucosal T cell that enables it to toggle between these responsive and unresponsive states. We recently identified that a chemical balance (called the redox potential) within the T cell regulates whether it can respond weakly or strongly to the outside world. We also showed that by switching the balance within a mucosal T cell to the alternate type this treated intestinal T cell now behaves more like a normal blood-derived T cell. This project will define the exact molecules in the T cell that causes this imbalance. We can then target this molecule for 'correction'and in the future determine whether this type of treatment is an effective therapy for mucosal infections, vaccine development, or mucosal autoimmune-like diseases.